1. Technical Field of the Invention
The present invention is in the field of automatic protective devices and pertains in particular to protection of electronic test equipment that performs low level measurement of the characteristics of electronic capacitors.
2. Background Art
In many areas of electronics it is necessary to measure the characteristics of passive electronic components. The ability to accurately pinpoint faulty or defective components has enormous economic value both for production line assembly and repair. Test equipment for measuring and testing of capacitors includes impedance measuring instruments, equivalent series resistance (ESR) meters, and capacitance meters.
However the testing of capacitors is problematic as compared to the testing of resistors. When testing capacitors, if the capacitor being measured is holding an electrical charge, the measuring instrument, other components or the capacitor itself may be damaged. In addition, there is the increased safety risk to personnel whenever dealing with charged capacitors that are not properly handled.
Personnel using such instruments must be sure to safely and completely discharge electronic capacitors before attempting a measurement of the capacitor""s characteristics. The discharging process can be time-consuming, hazardous to personnel, equipment and the meter and equipment. And, the discharge may also be incomplete. If personnel skip the discharge process or fail to complete the full discharge process, the capacitor can retain an electrical charge that can damage the measuring instrument and the capacitor itself when testing.
Some instruments such as the CapAnalyzer 88A employ a relay to first discharge the capacitor under test. Such arrangements are limited to the current carrying capacity of the relay contacts and the safe discharge current capability of the capacitor under test. The additional components add to the cost and size of the equipment. Testing of capacitors that exceed the discharge path limits the effectiveness of the tester. Finally, employing such a discharge process adds to the time required to perform the testing.
There have been several attempts in the prior art to address the aforementioned problems. A method and apparatus for testing electrolytic capacitors is described in U.S. Pat. No. 4,216,424 (""424). The ""424 reference discloses an Equivalent Series Resistance (ESR) measurement device for testing electrolytic capacitors in-circuit and includes a method of protection from damage due to charged capacitors. However, the ""424 method does not provide full protection from damage, since a large test capacitor with a large charge can still damage meters and equipment.
In U.S. Pat. No. 6,198,290 a method to detect defective capacitors is disclosed, wherein there is a method for measuring the value of capacitors in-circuit, but there is no protection against charged capacitors.
There is an in-circuit capacitor checker set forth in U.S. Pat. No. 6,054,864, wherein the capacitor test apparatus measures the value of capacitors in-circuit, and includes a user-operated switch for discharging the capacitors being tested.
Two related patents, U.S. Pat. No. 4,825,147 and U.S. Pat. No. 4,795,966, disclose an instrument and method for measuring the equivalent series resistance of a capacitor. The disclosed design offers little protection, and the device can be damaged by charged capacitors.
Many electronic measuring instruments, such as ESR meters, determine the characteristics of a capacitor by applying low-level alternating current or pulse signals to the capacitor under test. Low levels are used to permit xe2x80x9cin-circuitxe2x80x9d testing. If the test signal amplitude is below the threshold of semiconductor devices then those devices are not affected and the capacitor characteristics can be measured as if it were the only component in the circuit. The low level test signal must be passed unimpeded by the discharging/protecting device. The harmful charge on the capacitor under test is a direct current signal and must be discharged while minimizing the effects of that discharge on the measuring instrument and the capacitor under test.
What is needed is a device that permits the measuring instrument to accurately perform its function while automatically discharging capacitors under test in a manner that does not pass the charge to the measuring instrument, does not damage the capacitor under test and does so without unnecessary delay.
The invention is devised in the light of the problems of the prior art described herein. Accordingly it is a general object of the present invention to provide a novel and useful technique that can solve the problems described herein.
In an embodiment of the present invention, a system is provided for permitting electronic measurement equipment to accurately measure the alternating current and pulse characteristics of capacitors regardless of pre-existing charges on the capacitor under test.
In one embodiment, the measuring instrument is connected to the primary coil of a transformer having winding ratios of 1:1:1. The secondary windings of the transformer are connected to each other in opposing polarity through two diodes connected xe2x80x9cback to backxe2x80x9d, and the capacitor under test is connected across the secondary windings. Since the measuring instrument produces low level alternating current signals, the diodes are not conducting, so that the instrument is simply coupled to the capacitor under test through the transformer. This permits normal operation of the instrument.
If the capacitor under test has an electrical charge, the charge will cause one of the diodes to become conducting, and discharging current flows through both secondary windings of the transformer. These currents are nearly equal to each other so that the resulting flux in the transformer core will be nearly zero resulting in minimal coupling of the charge to the primary coil.
Once the capacitor under test is discharged sufficiently, the diode stops conducting and the remaining charge is dissipated through the single secondary winding. After the charge on the capacitor under test is completely removed the measuring instrument completes the measurement.
Another embodiment adds a separate discharging circuit to minimize the amount of discharging current that flows through the transformer windings. A low value blocking capacitor in series with the transformer secondary windings minimizes the current flow while a shunt resistor discharges the capacitor under test.
A further embodiment adds a pair of xe2x80x9cback to backxe2x80x9d diodes to the discharge shunt resistor. The purpose of these diodes is to decouple the measuring signal from the shunt resistor. Many ESR meters use low level signals to perform the measurement function, but are also able to measure high values of ESR. In such cases, the presence of the discharging shunt resistor could create a measurement error, and adding the decoupling diodes eliminates such error.
An object of the invention is an electronic apparatus for measuring performance of a capacitor under test using an Equivalent Series Resistance (ESR) meter, comprising a transformer having a primary winding coupled to the ESR meter and a pair of secondary windings coupled to the capacitor under test. There are a pair of opposing diodes in parallel and coupled to the pair of secondary windings in opposing polarity through the pair of diodes.
A further object is the electronic apparatus, wherein the apparatus is an external device coupling to the ESR meter on a first connector and coupling to the capacitor on a second connector. Alternatively, the apparatus is integrated within said ESR meter.
An additional object is the electronic apparatus, wherein the transformer has a winding ratio of 1:1:1. The device can also use a low value blocking capacitor in series with the secondary windings. The device can also have a first shunt resistance coupled in parallel to the capacitor under test. In addition, there can be a second shunt resistance coupled in series to the first shunt resistance, and an opposing pair of shunt diodes coupled parallel to the second shunt resistance.
Still other objects and advantages of the present invention will become readily apparent to those skilled in this art from the following detailed description, wherein I have shown and described only a preferred embodiment of the invention, simply by way of illustration of the best mode contemplated by me on carrying out the invention. As will be realized, the invention is capable of other and different embodiments, and its several details are capable of modifications in various obvious respects, all without departing from the invention.